Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust passage to an engine intake passage. By controlling EGR to provide a desired engine dilution, engine pumping work, engine knock, as well as NOx emissions may be reduced. For example, at partial throttle operating conditions, providing EGR to the cylinders of the engine allows for the throttle to be opened to a greater extent for the same engine load. By reducing throttling of the engine, pumping losses may be reduced, thus improving fuel efficiency. Further, by providing EGR to the engine, combustion temperatures may be reduced (especially in implementations where EGR is cooled prior to being provided to the cylinders). Cooler combustion temperatures provide engine knock resistance, and thus increase engine thermal efficiency. Further still, EGR reduces a combustion flame temperature that reduces an amount of NOx generated during combustion. In one example, during a combustion cycle, all EGR and fuel is provided to cylinders of an engine after intake valve opening.
However, the inventors herein have identified a potential issue with such an approach. For example, the amount of EGR provided to the engine cylinders may be limited by an engine dilution limit where combustion stability becomes degraded.
Thus in one example, some of the above issues may be at least partly addressed by a method comprising: at a first temperature and a first engine speed and load, supplying a first EGR amount to a cylinder; and at the first engine speed and load, as engine temperature increases from the first temperature to a second temperature, injecting a first fuel amount after exhaust valve closing and before intake valve opening and supplying a second EGR amount to the cylinder that is greater than the first EGR amount after intake valve opening.
By injecting an amount of fuel after exhaust valve closing and before intake valve opening, the fuel may interact with the hot/warm conditions in the cylinder. Such interaction causes at least some of the fuel to be converted into chemical radicals. Those chemical radicals then act as a catalyst to enhance combustion during compression and combustion strokes. In other words, the additional chemical radicals created by fuel injection after exhaust valve closing and before intake valve opening, lower engine dilution to facilitate stable combustion with an increased amount of EGR relative to an approach where fuel is merely injected after intake valve opening. The increased EGR concentration may lower the engine temperature to reduce emissions and increase efficiency and engine knock resistance.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.